The Fourth crusade:

The Fourth Crusade (1202–1204) was one of the most important crusades . It is also one of the hardest ones to understand and it is easily misunderstood, or at least is often understood in much too simplistic a manner. The Fourth Crusade was originally designed to conquer muslim jerusalem.
The main reason the Fourth Crusade was called was because the Third Crusade had failed to capture Jerusalem due the the lack of soldiers. Almost immediately the Pope began organising a new crusade. Still it took far longer than anticipate. Events were afoot in Constantinople that would cause the Fourth Crusade to take a dramatic turn away from Palestine and Jerusalem. The Fourth Crusade did not end up recovering Jerusalem, instead the Crusaders ended up attacking Constantinople and also kicking out the Byzantine Emperor and also putting one of their own in the ancient capital of Constantine.